Optical fiber technology requires precise characterization and control of various fiber properties during the process of drawing the fiber from a preform. For example, control of the glass dimensions is crucial to the waveguiding properties of the fiber, such as its dispersion, micro-bend losses and scattering losses. The coating thickness influences the bend loss properties, splicing and cabling. Detection of air lines within the fiber is important for loss and strength quality control. Fiber eccentricity leads to polarization mode dispersion (PMD). Thus, all of these quantities need to be monitored during the draw process, without perturbing the fiber or the process.
Over the years, fiber dimension measurements have been addressed using two techniques: (1) the “shadow technique” where a laser beam is scanned across a fiber and the change in transmission is analyzed to retrieve the fiber dimensions, and (2) the “forward scattering technique” which is an interferometric technique that analyzes the  interference between light transmitted through a fiber and light reflected by a fiber to determine the fiber diameter. Currently, the accuracy of these prior art techniques is limited to approximately 0.1 μm.
To date, fiber eccentricity is calculated by measuring the fiber dimensions is two orthogonal directions, using one of the two above-described techniques. However, in today's fibers, the differences in dimension in the two directions approaches approximately 100 nm, which is at the limit of currently available measurement techniques. At high data transfer rates, even a sub-100 nm difference results in significant PMD. Thus, there remains a need for a method and system for measuring fiber eccentricity, as well as other characteristics, with improved, nm scale accuracy.
Additionally, there are some properties that cannot be measured using the prior art techniques. For example, fiber-coating concentricity cannot be measured if the coating refractive index is less than the refractive index of the fiber itself, which is the case for at least one class of fibers currently defined as “hard clad silica” (HCS) fiber. In other cases, a dual polymer coating is applied simultaneously at the same physical location on the draw tower, and there is no technique that can be used to measure the thickness of the inner coating, where this inner coating plays a crucial role in determining the micro-bend loss properties of the fiber. It is also desirable to measure the refractive index of the fiber through the various stages in the draw tower, so as to determine parameters such as fiber temperature, stress and strain, where all of these properties can further affect the PMD.
The manufacture of optical fibers requires strict control of many parameters during the draw process in order to achieve the necessarily tight specifications on the refractive index profile. In particular, fiber tension during draw has been found to directly impact the index difference between various regions of the fiber and is therefore one of the most important draw parameters. Most prior art non-contact tension measurement gauges in use on draw towers today measure the mechanical vibration of the fiber after perturbing the fiber with a puff of air, considered to be a rather inaccurate and undesirable measurement technique. One prior art arrangement has been developed which measures the polarization-dependent side scatter from the fiber. This technique is nonlinear and considered to be inaccurate for large tensions, since its accuracy is dependent upon the fiber movement and ellipticity.
With the push towards increasing the rate and distance of data transmission on a single optical fiber, the quality of the optical fiber is of paramount importance. An improved ability to measure the various above-described fiber characteristics, including refractive index, is considered to be critical.